The testing of electronic circuits usually results in large quantities of data that need to be processed in order to detect and locate faults within the circuit under test. For any given circuit, there is a set of potential faults that may exist when the circuit is tested. Such set of potential faults along with their symptoms, called a dictionary, is a useful aid to the test operator in detecting and diagnosing faulty operation of the circuit. However, dictionaries for electronic digital circuits are often long, somewhat cumbersome to use, and may lead to misclassifications of faults. Therefore, it has been proposed to compress the output data of a circuit under test thus rendering such output signals more manageable. The goal of data compression is to characterize a digital bit stream using as few bits as possible and still preserve sufficient information to achieve detection and/or location of faults within a circuit. Such technique referred to as signature analysis, is described in an article by Ira H. Spector entitled "Logic-State and Signature Analysis Combine for Fast, Easy Testing" in Electronics, June 8, 1978, pages 141-145. As shown therein, a linear feedback shift register is fed by the sum modulo two of an incoming digital data stream and a specified fixed set of feedback signals from fixed stages of the register. Although this prior art signature analysis tester operates satisfactorily for its intended purpose, i.e., as a testing apparatus for digital circuits, such a testing apparatus utilizes a fixed length shift register as well as a fixed number of feedback taps. In other words, a certain fixed length shift register is dedicated to the testing apparatus and such apparatus is capable of only testing digital circuits.
Therefore, there exists a need for a flexible testing apparatus applicable to digital and analog circuits wherein a user or operator may selectively modify the physical configuration and processing time and mode of the testing apparatus.